Breathing frequency valves incorporated in the patient supply line typically have been controlled by signals emitted from a programmer to the valve so that the valve is open during only a certain portion, appropriately 22%, of a breathing cycle. During the remaining part of the breathing cycle, the valve must be closed. In some commonly employed respirators, the pressure in the gas line to the catheter which is introduced into the lungs is in the range of about 10.10.sup.3 -150.10.sup.3 Pa during the inspiration phase, that is, the phase during a respiration cycle when breathing gas is supplied to the patient. During the following expiration phase, the pressure in the gas line falls appreciably, so that when the expiration passages are completely free, it should have dropped to below about 2.10.sup.3 Pa after only half the respiration cycle. Occasionally, however, the expiratory passages can be blocked by a clot of phlegm or the like. In such circumstances, it is absolutely essential to switch off the respirator immediately, since otherwise the lungs would be over expanded by the frequent inspirations, possibly leading to a fatal condition.